Cannabinoids are the principal psychoactive component of cannabis, the most commonly used illicit drug in the United States. Going beyond the abuse issues there is also great interest in the possible medicinal use of cannabis and cannabinoids. Furthermore, manipulation of the endogenous cannabinoid system shows great therapeutic promise. While it is known that cannabinoids induce biological effects by engaging CB1 and CB2 receptors, strong experimental evidence suggests that additional cannabinoid receptors exist. We have recently confirmed that the orphan receptor GPR55 constitutes one such novel cannabinoid receptor. Specifically, we found that A9THC, JWH-015 (a synthetic cannabinoid) and methanandamide (a stable endocannabinoid analog) activate phospholipase C, increase intracellular calcium and stimulate p38 MAP kinase phosphorylation in GPR55-expressing HEK293 cells. GPR55 is expressed at high levels throughout the brain, immune system, adipose tissue and testis, as well as in neurons, astrocytes and microglial cells in culture, as assessed by RT-PCR. In this application we describe a plan to fully characterize GPR55 distribution and signaling, and determine its place in mediating the effects of plant-derived, synthetic and endogenous cannabinoids. We will do this by completing the following specific aims: 1. Determining the tissue and cellular distribution of GPR55. 2. Identifying the best pharmacological tools to study GPR55 and elucidating its signaling pathways. 3. Ascertaining how GPR55 regulates neuronal, microglial, and adipocyte functions. The completion of these specific aims will allow us to evaluate the role of GPR55 as an additional cannabinoid receptor, and will provide the necessary pharmacological and molecular tools required to determine its involvement in mediating cannabimimetic effects.